
where \(\rho e^{\jmath \phi }\)  is an unknown complex gain, modeled with \(\phi \sim \mathcal {U}[0,2\pi )\)  and [1]}
\(\rho = \sqrt{\frac{G_{t} G_{r} A \lambda ^2}{64\pi ^3}}\frac{1}{ \Vert \mathbf {p}_{\text{tx}}-\mathbf {p}_{\text{ris}}\Vert  \Vert \mathbf {p}_{\text{rx}}-\mathbf {p}_{\text{ris}}\Vert },\) 
